1. Field of the Invention
This invention relates to a safety device for stopping a vertically moving body in an emergency and, in particular, to a brake shoe for an elevator safety device for stopping an elevator car in an emergency.
2. Description of the Related Art
FIG. 6 schematically shows the construction of a conventional elevator safety device disclosed, for example, in Japanese Patent Laid-Open No. 56-155178, the teachings of which are hereby incorporated by reference. Referring to FIG. 6, a pair of car guide rails 1 are provided on a hoistway wall (not shown). A car 2 is hung to one end of a hoisting rope 3 so as to be hoisted and lowered in the hoistway space along the guide rails 1. A safety device 4 is attached to the bottom of the car 2. The safety device 4 comprises two pairs of brake shoes or braking elements 5 for being brought into contact with and separated from the guide rails 1 and pressure springs 6 for pressing the braking elements 5 against the guide rails 1. The guide rails 1 are respectively held between the braking elements 5 of each pair.
Next, the operation of the above safety device will be described. When the car 2 starts to descend at an undesirable speed which is in excess of the rated speed as a result of accident, breakdown, breakage of the rope 3, etc., the braking elements 5 are pressed against the guide rails 1, and the car 2 is decelerated and brought to a stop by a frictional force generated between the braking surfaces (the sliding surfaces) 5a of the braking elements 5 and the guide rails 1. A braking force F obtained by the four braking elements 5 can be expressed by the following formula: EQU F=4 .mu.P,
where .mu. represents the coefficient of friction and P represents the pressing force of the pressure springs 6.
Thus, a braking performance of the safety device 4 is determined by the product of the coefficient .mu. of friction between the braking elements 5 and the guide rails 1 and the pressing force with which the braking elements 5 are pressed against the guide rails 1. Therefore, the safety device 4 could be realized as a small-sized, high-performance device by increasing the coefficient .mu. of friction and the pressing force. Before the pressing force P can be increased, the braking elements 5 must be formed of a material having a high withstanding pressure per unit area. As to the coefficient of friction, it can be increased by providing a large number of projections on the braking surface 5a of each braking element 5 so as to enable the braking element 5 to be more readily engaged with the guide rails 1. However, this results in a reduction in the effective contact area between the braking surfaces 5a and the guide rails 1. Thus, also from this point of view, it is important to select a material having a high withstanding pressure for the braking elements 5.
The withstanding pressure of the braking elements 5 is determined by the frictional fusing limit of the braking surfaces 5a when they are pressed against the guide rails 1 and the yield strength of the material of the braking elements 5. When deformation, fusion, etc. are generated on the braking surfaces 5a of the braking elements 5, the frictional force becomes unstable and the braking surfaces 5a become more liable to be abraded and worn out. In particular, in the case that the car 2 moves at high speed, the abration may become so intense that braking cannot be effected. Thus, to obtain the requisite braking performance, the braking elements 5 should be used within a range where no excessive plastic deformation or fusion is generated on the braking surfaces 5a. Regarding the guide rails 1, they are less subject to fusion than the braking surfaces 5a since those sections of the guide rails 1 which are in contact with the braking surfaces 5a are gradually shifted downwards as the car 2 descends, so that the accumulation of frictional heat is less than on the braking surfaces 5a. Therefore, by enhancing the heat resistance of the braking elements 5, deformation, fusion, etc. of the braking surfaces 5a can be prevented, thereby making it possible to realize a braking operation with high bearing pressure.
In the conventional safety device for elevators as described above, it is possible to attain an improvement in braking performance by enhancing the heat resistance of the braking elements 5. However, in the case of recently developed super-high-speed, high rise elevators running at a speed of 400 m/min or more, or even at a speed of as high as 1500 m/min, which have already been put into practical use, the braking distance is 10 m or more, so that the frictional heat of the braking surfaces 5a during braking exceeds 1000.degree. C., as shown in FIG. 7. In such high rise elevators, a sufficient heat resistance for the braking elements 5 is not to be expected with the conventionally adopted materials, and the requisite level of safety and reliability cannot be obtained.
FIG. 8 shows the relationship between temperature and hardness in various materials. When the temperature of a material rises to such a degree as to cause a marked reduction in hardness, deformation, fusion, etc may occur in the materials except ceramics. As shown in FIG. 8, when compared with the other materials, ceramics are more capable of maintaining a sufficient degree of hardness even at a high temperature and less subject to the occurrence of deformation or fusion. However, ceramics have a rather low level of toughness, so that, if the braking elements 5 are formed of a ceramic material, there is a danger that cracks are generated in the braking elements 5 or the braking surfaces 5a are damaged by various shocks during operation and the shock when they hit against the guide rails 1 at the start of theirs operation, resulting in brake failure.